The present invention relates to rearview mirrors, and more particularly to rearview mirrors capable of automatically changing their reflective states in response to varying light levels.
High light levels reflected from a rearview mirror into the eyes of a vehicle operator during night driving can seriously impair the operator's forward vision. Light scatters within the eye causing a "veil" or "curtain" of light over the scene. The driver therefore has a reduced ability to detect objects which are dimly lit or have little contrast with the background. This situation is known as disability glare. The driver is normally unable to detect this loss of visibility since there is no physical sensation associated therewith. At higher reflected light levels, discomfort glare occurs, resulting in an unpleasant physical sensation in the driver's eyes.
Manually actuated rearview mirrors have been developed which include "day" and "night" settings. These day/night mirrors include a mirror prism and a mechanism for changing the angular orientation of the prism. In the "day" setting, the mirror prism is angularly set to provide approximately 80% reflectance of the rearward image into the driver's eyes. In the "night" setting, the prism is angularly set to provide only approximately four percent of the full reflectance of the rearward image into the driver's eyes. These mirrors suffer several disadvantages. First, the mirrors must be manually actuated or switched between the day and night settings. This can be a distraction when the driver should be concentrating his full attention on the operation of the vehicle. Second, when in the night setting, the mirror reduces the driver's ability to obtain information from the mirror because of the greatly reduced reflectance. Third, when in the day setting, quickly-appearing bright lights in the mirror can seriously impair the driver's vision.
"Automatic" day/night rearview mirrors have been developed which automatically switch the mirror prism between full and partial reflectance conditions in response to sensed light levels. One such mirror is illustrated in U.S. Pat. No. 4,443,057, issued Apr. 17, 1984 to Bauer et al, entitled AUTOMATIC REARVIEW MIRROR FOR AUTOMOTIVE VEHICLES. The Bauer mirror includes a forward light sensor providing a signal representative of light forward of the mirror, a rear light sensor providing a signal representative of light rearward of the mirror, and a control circuit responsive to the independent forward and rear signals to control the mirror prism position. When the rear sensor signal exceeds a value dependent on the conditioned independent forward signal, the mirror prism is switched to the partial reflectance mode. Time-averaging filters filter the output of both the forward and rear light sensors. These filters prevent light "spikes" from momentarily changing the reflective state of the prism. This reduces excessive actuation of the mirror between its full reflectance and partial reflectance modes in response to changing light conditions of relatively short duration, such as passed street lights The time constant provided by the forward signal filter is longer than the time constant provided by the rear signal filter so that (1) the ambient light reference level is more slowly affected by changing light levels to provide a reliable reference level and (2) the rear signal is adequately responsive to changes in rear light levels.
The Bauer control circuit is not without its drawbacks. First, the ambient light reference level does not adequately track the changing sensitivity of the driver's eyes which changes due to the varying ambient and rear light levels experienced under typical driving conditions. One cause of this is that the independent forward signal is filtered or time averaged using a single time constant. However, it is known that the human eye adapts more quickly to increasing light levels than it does to decreasing light levels. Second, the signal used to compute the ambient light reference level is dependent only on the light levels forward of the mirror housing and is not dependent on other significant light levels within the driver's field of view. In summary, the mirror prism is sometimes actuated when not necessary to meet the sensitivity of the human eye and sometimes not actuated when necessary to meet the sensitivity of the human eye.
The ideal rearview mirror would provide maximum image brightness at all times consistent with the need to reduce rear glare to acceptable levels, thus maintaining optimum forward and rearward visibility. To achieve this result, the rearview mirror must calculate the dynamic glare tolerance of the driver's eyes, which is an indication of the glare that can be directed into the driver's eyes without causing disability glare or discomfort glare.